1. Field of the Invention
The present invention relates to an improved solvent having a low toxicity, a low combustibility and a low capability of depleting an ozonosphere and a polyolefin solution using said solvent. Further, the present invention provides a process for producing an improved process for producing a three-dimensional fiber of a polyolefin having an excellent strength and spreadability for use in an nonwoven fabric sheet through the use of said solvent and said solution.
Namely, the present invention relates to an improved solvent which is non-toxic and non-combustible and very safe when used, and a polyolefin solution and an improved flash spinning process using said solvent. The solvent according to the present invention can be applied not only to a polyolefin solution for flash spinning but also to a cleaning agent, a foaming material, a gas for the production of a hollow filament, and a reaction solvent, etc.
2. Description of the Related Art
A process for producing a polyolefin fiber having a plexifilamentary structure is known as a flash spinning process. The flash spinning process is a well known spinning process which comprises adding a polyolefin to an organic solvent also known as a liquefied gas, preparing a polyolefin solution under high temperature and high pressure conditions, passing the solution through a pressure let-down orifice to once lower the pressure of the solution to bring about a phase separation, and passing the opaque solution through a spinneret to inject the solution into an atmosphere where the temperature and pressure are room temperature and normal pressure, respectively, to thereby form a fiber having a three-dimensional structure.
This spinning process is described in, for example, U.S. Pat. Nos. 3,081,519, 3,227,794, 3,227,784, 3,467,744, 3,564,088 and 3,756,411, EP 285670 A1, EP 321567 A1, EP 357364 A2, Japanese Examined Patent Publication (Kokoku) No. 40-28125, Japanese Examined Patent Publication (Kokoku) No. 42-19520, Japanese Unexamined Patent Publication (Kokai) No. 62-33816 and Japanese Unexamined Patent Publication (Kokai) No. 63-50512.
The fiber prepared by this flash spinning process is classified into a short fiber-like material and a three-dimensional fiber material. The former is used as a synthetic pulp, and the latter is used as a nonwoven fabric sheet. The nonwoven fabric sheet is generally known as a synthetic paper. The largest feature of this product resides in a good water resistance, high strength and light weight, and freedom from fuzzing. This is highly appreciated in the art, and this nonwoven fabric is applied to envelopes for air mail, sleeves for floppy disks, bags for deoxidizers, bags for desiccants, medical sterilizing bags; clothes for preventing dewing caused by thermal insulation of buildings, working wear for working in nuclear power generation, working wear for asbestos, and working wear for safety and protection, etc. A three-dimensional fiber which has a high strength and is highly spreadable is indispensable to the production of products having these features. This is because a uniform sheet which is dense and permeable to air can not be produced without the use of such a fiber.
The above-described nonwoven fabric sheets are already commercially available as Tyvek.RTM. from DuPont in U.S.A., and Luxer.RTM. from the applicant of the present invention.
The solvent for the polymer used in the flash spinning process should have the following properties. This is also shown in U.S. Pat. No. 3,081,519. (1) The boiling point of the solvent is at least 25.degree. C. below the melting point of the polymer used; (2) the solvent is inert to the polymer under spinning conditions; (3) the solvent is a good solvent for the polymer under temperature and pressure conditions suitable for the preparation of a polymer solution; (4) the solvent dissolves only 1% of the polymer when the temperature is below the boiling point of the solvent; and (5) the solvent can immediately give rise to a phase separation at the time of spinning to form a phase consisting essentially of a polymer, and the separated polymer phase is substantially free from the solvent.
Specific known examples of the solvent include aromatic hydrocarbons such as benzene, toluene, aliphatic hydrocarbons such as butane, pentane, hexane, heptane and octane and their isomers and homologues, alicyclic hydrocarbons such as cyclohexane, unsaturated hydrocarbons, halogenated hydrocarbons such as methylene chloride, carbon tetrachloride, chloroform, ethyl chloride and methyl chloride, alcohols such as ethanol, methanol and hexafluoroisopropanol, esters, ethers, ketones, nitriles, amides, fluorochlorinated aliphatic hydrocarbons such as trichlorofluoromethane, 1,1,2-trichloro-1,2,2-trifluoromethane, sulfur dioxide, carbon disulfide, nitromethane, water and various liquid mixtures of the above-described solvents.
An optimal solvent is properly selected from these solvents after various conditions for the spinning process used and the kind of polymers used are taken into consideration. Trichlorofluoromethane and 1,1,2-trichloro-1,2,2-trifluoroethane which has a high capability of dissolving the polymer and an excellent spinnability and are noncombustible and nontoxic are favorable as the solvent for the flash spinning process of a polyolefin. Among others, trichlorofluoromethane is the best solvent.
In the flash spinning, to inject a polymer solution under high temperature and high pressure into the air to gasify the solution, the solvent should have a low boiling point, remain undecomposed even at a high temperature, have a lipophilic property sufficient for dissolving the polyolefin, and have at least a low toxicity and be fire-resistant. Specifically, in the flash spinning, the solvent is gasified to separate the solvent from the polymer, and the gasified solvent is recovered and liquefied by compression with cooling. Therefore, the flash spinning is conducted in an extensive, sealed space. This is because a gasified solvent can not be recovered when the flash spinning is not conducted in such an extensive space. The size of the sealed space is, for example, as large as 2000 m.sup.3. The filling of such an extensive space with a combustible gas increases the possibility of fire and explosion, and therefore is, very dangerous, which makes it substantially impossible to use a combustible gas as a solvent.
In general, a corona discharge device or a high-voltage destaticizer is contained in the sealed space and can be an ignition source of the combustible gas. This further makes the use of a combustible gas unfavorable. Further, various facilities such as a metallic conveyor for the formation of a nonwoven fabric, a corona discharge device and a spinning head are provided in the sealed space, and this makes it unavordalbe that workers must enter the sealed space for repair and maintenance work. Further, the outlet port for the formed nonwoven fabric sheet is non-contact sealed, and the gas within the sealed space always leaks into the work section. Therefore, when the solvent is toxic, it cannot be used as a solvent for flash spinning. For this reason, trichlorofluoromethane, which is noncombustible and nontoxic, has been regarded as the only solvent useable for the flash spinning process.
In recent years, however, it has been found that a wholly halogenated hydrocarbon wherein all the hydrogen atoms are substituted with chlorine and fluorine is a particular flon (chlorofluorocarbon also known as "CFC") having a very high capability of depleting ozonosphere. It has been decided that the production of a CFC be prohibited by A.D. 2000, from the viewpoint of a protection of the environment. It is a matter of course that the production of trichlorofluoromethane, 1,1,2-trichloro-1,2,2-trifluoroethane, etc. as particular flons also will be prohibited, and these flons become commercially unavailable. Therefore, trichlorofluoromethane will become unable to be utilized as a solvent in the flash spinning process of a polyolefin.
Under these circumstances, a flash spinning process wherein a new solvent is used without the use of trichlorofluoromethane as a particular flon has been already proposed.
Specifically, U.S. Pat. No. 5,032,326, EP 0357381A2 and Japanese Unexamined Patent Publication (Kokai) No. 2-139408 disclose a flash spinning process wherein use is made of a mixed solvent comprising methylene chloride and an alternative flon, for example, chlorofluoromethane, 1,1,1,2-tetrafluoroethane, 1,1-difluoroethane, 1,1,1,2-tetrafluoro-2-chloroethane or 1-chloro-1,1-difluoroethane. Further, U.S. Pat. Nos. 5,081,177, 5,023,025, EP 0 361684A1, Japanese Unexamined Patent Publication (Kokai) No. 2-160909 disclose a spinning process wherein use is made of 1,1-dichloro-2,2,2-trifluoroethane, 1,2-dichloro-1,2,2-trifluoroethane, 1,1-dichloro-2,2-difluoroethane, 1,2-dichloro-1,1-difluoroethane or 1,1-dichloro-1-fluoroethane. Further, EP 0407953A2 discloses a spinning process wherein 1,1-dichloro-2,2,2-trifluoroethane, 1,2-dichloro-1,2,2-trifluoroethane or the like is used as a solvent for polypropylene. Further, EP 357364A4 and Japanese Unexamined Patent Publication (Kokai) NO. 3-76809 disclose a process wherein spinning is conducted through the use of methylene chloride and carbon dioxide. Further, EP 0414498A2 and Japanese Unexamined Patent Publication (Kokai) No. 3-152209 discloses a process wherein use is made of a mixed solvent comprising a water-containing organic solvent. Further, EP 431801 discloses a process wherein spinning is conducted through the use of carbon dioxide and water.
Japanese Unexamined Patent Publication (Kokai) No. 4-185708 discloses a spinning process using 1,1-dichloro-2,2,3,3,3-pentafluoropropane and/or 1,3-dichloro-1,2,2,3,3-pentafluoropropane mixed with hydrocarbon derivative.
All of the above-described proposed flash spinning processes, however, give rise to drawbacks when spinning is conducted through the use of a polyolefin.
For example, the mixed solvent comprising methylene chloride and an alternative flon proposed in U.S. Pat. No. 5,032,326 may be tentatively used as a solvent for use in a laboratory, but, it cannot be used as an industrial solvent. This is because methylene chloride is highly toxic and carcinogenic. TLV (threshold limit values of airbone contaminants) established by ACGIH (American Conference of Governmental Industrial Hygienists) is known as an index for indicating the degree of toxicity. The TLV of trichlorofluoromethane is 1000 ppm, but the TLV of methylene chloride is as low as 50 ppm. Therefore, it is apparent that methylene chloride has a higher toxicity than trichlorofluoromethane. Further, methylene chloride is registered as a carcinogenic substance. This clearly suggests that methylene chloride and its mixed solvent cannot be used on a commercial scale as a solvent for flash spinning. Further, the use of methylene chloride involves a problem from the viewpoint of properties required at the time of spinning. The reason for this is that the heat of evaporation of methylene chloride is 78.7 cal/g which is much larger than that of trichlorofluoromethane, i.e., 43.5 cal/g. This means that the use of methylene chloride as a solvent causes a spun yarn to be liable to be damped by the residual solvent. As described also in Japanese Unexamined Patent Publication (Kokai) No. 3-76809, the damped yarn is liable to adhere to and wind around a roller used for pressing the yarn into a sheet structure, so that a no nonwoven fabric sheet having a commercial value cannot be produced, which makes it impossible to produce the nonwoven fabric sheet on a commercial scale. For this reason, when methylene chloride is used as a solvent, it is necessary to allow a gas having a low boiling point to exist together with the methylene chloride solvent, for accelerating the evaporation of methylene chloride remaining in a fiber as spun, and at the same time, to previously enhance the polymer concentration of a spinning solution. The enhancement of the polymer concentration is effective for increasing the amount of occurrence of heat of solidification at the time of flashing of the polymer solution to accelerate the drying of a spun yarn through the utilization of the heat. In this case, the yarn becomes damped when either the use of a gas having a low boiling point is omitted or the enhancement in the polymer concentration is low. However, when the polymer concentration of the solution is enhanced, the spread state of the spun yarn becomes so poor that the quality of the sheet products lowered. Therefore, also from the viewpoint of spinnability, it is understood that methylene chloride cannot be used.
The proposal in U.S. Pat. No. 5,081,177 wherein use is made of an alternative flon has a problem. Specifically, studies in recent years have revealed that 1,1-dichloro-2,2,2-trifluoroethane and its isomers give rise to a tumor in rats, although it is benign. Further, since these alternative flons are a poor solvent for a high-density polyethylene, which is a typical example of a polyolefin, they cannot dissolve the high-density polyethylene when used alone. For this reason, to improve the solubility, a technique wherein a hydrocarbon, methylene chloride, etc. are used as a co-solvent is simultaneously disclosed. However, even when the alternative flon is allowed to exist together with the hydrocarbon, methylene chloride or the like, since the proportion of the alternative flon in the solvent is high and 50% or more, the property that the alternative flon is a poor solvent for the high-density polyethylene strongly remains, so that it is difficult to dissolve the high-density polyethylene. Therefore, a problem that the dissolution rate is low remains unsolved. Thus, 1,1-dichloro-2,2,2-trichloroethane and its isomers have problems of the toxicity and the dissolution of the high-density polyethylene. Similarly, 1,1-dichloro-2,2-difluoroethane and its isomers have problems of the toxicity (toxicity against genital organs) and the dissolution of the high-density polyethylene. On the other hand, 1,1-dichloro-1-fluoroethane and its isomers dissolve the high-density polyethylene and provide a good yarn even when used alone. These solvents, however, are highly liable to thermal decomposition. Even when dissolution is conducted by means of an extruder, they are liable to thermally decompose with evolution of a large amount of hydrogen chloride or hydrogen fluoride to give a halogenated oligomer. These decomposition products give rise to serious problems such as coloring of a product and corrosion of a spinning apparatus. Further, 1,1-dichloro-1-fluoroethane as the solvent has an ozone depletion potential (ODP) exceeding 0.1, i.e., unfavorably has a high capability of depleting ozone. Therefore, 1,1-dichloro-1-fluoroethane and its isomers cannot be used as a solvent for flash spinning because they are highly liable to thermal decomposition and have a high capability of depleting ozone.
The technique proposed in EP 357364A4 cannot be used as a solvent for flash spinning for the reasons set out above because methylene chloride is used as the solvent.
The spinning process proposed in EP 431801 wherein flash spinning is conducted through the use of a solvent comprising carbon dioxide and water can be applied to a particular polyolefin having a high hydrophilicity as a comonomer component. Since, however, this solvent has a poor capability of dissolving general polyolefins, i.e., polyethylene and polypropylene, it is substantially impossible to prepare a fiber having favorable properties. Further, in this method, it is practically necessary to use a surfactant in combination with the solvent. This not only renders the process complicated but also causes the surfactant to remain in the resultant fiber to lower the practical properties.
Further, in a technique proposed in EP 0414498A2 wherein use is made of a mixed system comprising a water-containing organic solvent, since the solvent used is highly combustible, this technique cannot be used.
The spinning process using 1,1-dichloro-2,2,3,3,3-pentafluoropropane and/or 1,3-dichloro-1,2,2,3,3-pentafluoropropane mixed with hydrocarbon derivative proposed in Japanese Unexamined Patent Publication (Kokai) No. 4-185708 has a drawback due to use of the hydrocarbon derivative having a flammability and a high toxicity. Accordingly, this process cannot be applied to a practical production.
Thus, all the solvents alternative to trichlorofluoromethane for flash spinning proposed up to now apparently have an unsolved problem, and no satisfactory solvent which can be used instead of trichlorofluoromethane has been proposed in the art.